The invention is based on a priority application EP 03 291 570.4 which is hereby incorporated by reference.
The invention is related to a parametric amplifier connected to a pump laser with a control device for adapt the pump laser. The invention is also related to a transmission system using this parametric amplifier. In addition the invention comprises a method for controlling the pump laser source in an iterative process.
Optical amplifiers of the type in which the amplitude of electric field of light is directly amplified are applicable to the following uses in the optical fiber transmission system and on the optical amplifiers of this type is being made in various areas:—By increasing the output of a light source of the signal light in an optical transmitter, the transmission distance can be increased. When the optical amplifier is used for the light source of local light in an optical receiver on a coherent optical wave communication system, the reception sensitivity can be improved.                By performing optical amplification in the stage immediately before the photoelectric conversion stage, the reception sensitivity can be improved.        By the direct amplification of light, as compared with the method in a conventional optical repeater in which a light signal is once photo-electrically converted into an electric signal and then the electric signal is amplified, it becomes possible to make the repeater itself smaller in size and also to increase the repeater-to-repeater distance.        
Parametric amplifcation is a well known phenomenon in materials providing X(2) nonlinearity . However, parametric amplification can also be obtained in optical fibers exploiting the X(3) nonlinearity. New high power light sources and optical fibers with a nonlinear parameter 5–10 times higher than for conventional fibers as well as the need of amplification outside the conventional Erbium band has increased the interest in such optical parametric amplifiers (OPA). The fiber based OPA is a well-known technique offering discrete or “lumped” gain using only a few hundred meters of fiber. It offers a wide gain bandwidth and may in similarity with the Raman amplifier [be tailored to operate at any wavelength]. An OPA is pumped with one or several intense pump waves providing gain over two wavelength bands surrounding the single pump wave or, in the latter case, the wavelength bands surrounding each of the pumps. As the parametric gain process do not rely on energy transitions between energy states it enable a wideband and at gain profile contrary to the Raman and the Erbium doped fiber amplifier. The underlying process is based on highly efficient four photon mixing (FPM) relying on the relative phase between four interacting photons. An very important application is the possibility of in-line amplification with an ideal noise figure of 0 dB. This should be compared to the quantum limited noise figure of 3 dB for standard phase-insensitive amplifiers. For the phase-insensitive OPA, two photons at one or two pump wavelengths with arbitrary phases will interact with a signal photon. A fourth photon, the idler, will be formed with a phase such that the phase difference between the pump photons and the signal and idler photon satisfies a phase matching condition. For the phase-insensitive OPA the requirements for its implementation are substantially relaxed and it still offers the important properties of high differential gain, optional wavelength conversion and operation at an arbitrary wavelength. As the Kerr effect, similarly to the Raman process, relies on nonlinear interactions in the fiber, the intrinsic gain response time for an OPA is in the same order as for the Raman amplifier (a few femtoseconds). This prevents in many, but not all cases, the amplifier from operating in a saturated mode.
The calculation shows that for the perfect phase matching case, the parametric gain is approximately exponentially proportional to the applied pump power. A very simple expression for the OPA peak gain may be obtained as:    GdB=10 log10[¼exp(2γPpL)]=PpLSP−6, where    SP=10 log10[exp(2)]γ≈8.7γ is introduced as the parametric gain slope in    [dB/W/km], with    γ=2πn2/λAeff as non linear coefficient and    n2 the fiber nonlinear parameter and    Aeff the effective modal area of the fiber, with    Pp as pump power and    L as length of the fiber.
The amplifier bandwidth may be defined as the width of each gain lobe surrounding λp. From the equitation it may be observed that the amplifier bandwidth for a fixed γPpL will increase with decreasing L as the reduction in Gs with respect to λ will be “accelerated” by the longer fiber length. On the other hand, since λPp increases as L decreases, the peak gain wavelength will be pushed further away from λp.
Another factor to take into account for the OPA gain bandwidth is the fact that λ0 the zero-dispersion in a real fiber is slightly distributed along the fiber length. This will flatten the resulting gain bandwidth compared to a fixed λ0 wavelength but will decrease the peak gain. By deliberately introducing a λ0 variation in the fiber it has been shown that a flat, broadband operation may be achieved.
Summarizing, key parameters such as high pump power, high non linearity coefficient, a short fiber length, a pump wavelength close to the zero-dispersion wavelength, and a low dispersion slope are identified for achieving a high gain and a wide bandwidth in single pumped fiber optical parametric amplifiers.
Normally the pump power is controlled in a parametric amplifier. One example of controlling the pump power and the gain of an optical amplifier is disclosed in U.S. Pat. No. 6,417,965. The optical amplifier control system provides real time control of an optical amplifier in response to an analog signal having a large dynamic range.
In real fiber transmission systems the dispersion varies along the fiber and with any fiber piece. In a result only an average measurement of the zero dispersion is possible. Since the parametric gain depends on every local dispersion the resulting parametric gain is not easily to foreseen. The prior art solution is to try for any fiber piece several pump sources to optimize the gain spectrum the best adapted to the application. With a fixed pump wavelength the parametric gain spectrum is also fixed. Increasing the pump level will than also influence the gain spectrum without further possibilities to adapt.